Printed wiring board (PWB) manufacturing processes have developed and changed at a rapid rate due to the increased demand for enhanced performance. The demand for enhanced performance is due, for example, to higher circuit densities, increases in board complexities and an increase in the cost of environmental compliance. Various types of final finishes are used on PWBs, and the selection of a final finish is typically dependent on the requirements that the board must ultimately meet. Surface circuits on PWBs usually include copper and copper alloy materials that generally must be coated to provide good mechanical and electrical connections with other devices in the assembly.
Typically, the coating on the circuits is called the surface finish. The circuits include non-contact areas and contact areas. The finish applied to the non-contact areas is called the non-contact finish and the finish applied to the contact areas is called the contact finish. Non-contact areas include wire-bonding areas, chip attach areas, soldering areas and other non-contact areas. Non-contact and contact finishes must meet certain different requirements. For example, non-contact finishes must provide good solderability, good wirebonding performance and high corrosion resistance. On the other hand, contact finishes must provide high conductively, high wear resistance and high corrosion resistance. To meet these different requirements, different coatings have been developed for non-contact finishes and contact finishes
Some typical non-contact finishes include (i) hot air solder level (HASL) coating, (ii) electroless nickel/immersion gold (ENIG), (iii) organic solderability preservatives (OSP), (iv) organo-metallic coatings, such as organo-silver, where the organic is co-deposited with the silver, (v) immersion silver (ImAg) and (vi) immersion tin (ImSn). These final finishes were generally developed to ensure a good solderable surface for component assembly during the board assembly process. A typical contact finish may include an electrolytic nickel coating with an electrolytic hard gold layer (gold-nickel or gold-cobalt alloys containing less than about 0.3 weight percent of nickel or cobalt) on top. To coat any of the above non-contact finishes on the non-contact areas or to coat the finish on the contact areas requires considerable processing steps and a considerable amount of time which potentially decreases production yields and also increases costs.
The PWB industry has begun to evaluate alternative surface finishes. There is a high demand for a multi-purpose finishes that can be used for both non-contact areas and contact areas and that can replace tin-lead in the PWB finishing processes, to lower environmental concerns and to make electronic circuit manufacturing “greener.” To be qualified, such a finish should provide high etch resistance, good solderability, good wirebonding performance, high conductivity, high wear resistance, high corrosion resistance/low porosity, coplanarity (uniform thickness distribution), integrity after exposure to soldering temperatures, ability to integrate into current manufacturing processes, have long storage life, and environmental safety.
Some known production methods utilize high-lead solder materials. Although most high-lead solders are relatively inexpensive and exhibit desirable properties, the use of lead in die attach and other solder materials has come under increased scrutiny from an environmental and occupational health perspective. Therefore, various approaches have been undertaken to replace lead-containing solder with lead-free materials.
The lead free final finishes most commonly applied today to printed circuit boards and other similar devices are organic solderability preservatives (OSP), immersion silver (ImAg), electroless nickel/immersion gold (ENIG), and immersion tin (ImSn) which were generally developed to ensure a good solderable surface for component assembly during the board assembly process. Examples of OSP compositions are described for example in U.S. Pat. No. 5,858,074 to Cole et al., U.S. Pat. No. 6,635,123 to Cavallotti et al. and U.S. Pat. No. 6,815,088 to Cavallotti et al., the subject matter of each of which is herein incorporated by reference in its entirety.
Corrosion resistance, particularly in harsh industrial environments, was not a design criterion. The effects of environmental pollutants on inducing failures in electronic products are far more serious in the lead-free world. With the transition to lead-free, two key factors are changed which increased the vulnerability of circuitry to environmental corrosion. First, HASL-alternative coatings, which rely on very thin deposits, are not especially robust as defenses to pollution. Secondly, the increased temperatures at which these coatings are soldered often detract from their ability to protect the circuitry over long periods of post assembly exposure. The printed circuit board industry has identified several cases of failure due to environmental conditions leading to corrosion, including a particular type of corrosion termed “creep corrosion.”
Accelerated creep corrosion has been observed on these finishes, particularly in sulfur-bearing environments, such as paper mills where sulfur is used in the bleaching process, tire manufacturing plants, which utilize a rubber vulcanization process, and in automotive prototype design studios, where the modeling clay that is used can contain in excess of 40% elemental sulfur, by way of example and not limitation. Where this occurs, short circuits may occur due to growth of corrosion product leading to system failure.
The inventors of the present invention have determined that the use of a modified OSP-type coating composition in combination with an emulsion polymer exhibits improved creep corrosion resistance for metal finishes on printed wiring boards, such as silver final finishes.